Regarding your original question about the wrong week, read that "%W" definition carefully. Using "%W", the first week of 2019 didn't start until January 7th, because the 7th was the first Monday of the year. So December 31 2018 through January 6 2019 was the 53rd week of 2018 (or week zero of 2019, I forget which... maybe both). You might consider using "%V" instead of "%W". "%V" uses the ISO 8601 standard, which uses a different method of calculating week numbers. Using "%V", the first week of 2019 was from December 31 to January 6. 2019 is an example of a year in which "%W" and "%V" return completely different values for the entire year.

P.S.: The Rainmeter docs are incorrect for %V, in that there is never a week zero in ISO 8601 standard, weeks always start with week 1.

The ISO 8601 week and week-based year produced by %V, %g, and %G, uses a week that begins on Monday, where week 1 is the week that contains January 4th, which is the first week that includes at least four days of the year. If the first Monday of the year is the 2nd, 3rd, or 4th, the preceding days are part of the last week of the preceding year. For those days, %V is replaced by 53, and both %g and %G are replaced by the digits of the preceding year.

Or put another way:

Week 19 is from Monday May 6, 2019 until (and including) Sunday May 12, 2019. Week number according to the ISO-8601 standard, weeks starting on Monday. The first week of the year is the week that contains that year's first Thursday (='First 4-day week'). The highest week number in a year is either 52 or 53.